Conventionally, there is known a small-sized battery charger having, as shown in FIG. 3, a handle 1 arranged for rotation, a rotation transmitting wheel 2 to rotate in synchronism with the handle 1, an intermediate wheel 3 and a power-generating gear 4 to transmit the rotation of the rotation transmitting wheel 2, a generator 5 to convert the rotational energy of the power-generating gear 4 into an electrical energy that is non-invertable in electric polarity depending on the rotation, and wherein the generator has one output connection OUT1 electrically connected to a positive side of an electricity storing means 6 (e.g. a secondary battery or a capacitor) through a conductive material 10a and a plus connection 8 and another output connection OUT2 electrically connected to a negative side of an electricity storing means 6 through a conductive material 10b, an electrical rectifying element (e.g. a diode) and a minus connection 7, so that a rotational kinetic energy applied to the handle 1 is converted into an electrical energy and stored in the electricity storing means 6.
Incidentally, the generator 5, if rotated at a constant rotational speed in a positive direction, has a terminal voltage with a characteristic increased in potential at a plus side relative to GND as shown in a solid line "a" in FIG. 4. If rotated at a constant rotational speed in a reverse direction, the characteristic has a potential decreased at a minus side relative to GND as shown in a solid line "b" in FIG. 4.
This conventional small-sized battery charger has an electrical circuit configuration as shown in FIGS. 5(a) and (b). Explanations will be made on the principle of electricity charging based on an operational flowchart shown in FIG. 6.
First, if the handle 1 is rotated leftward as shown in FIG. 5(a), the rotation of the rotation transmitting wheel 2 is transmitted through the intermediate wheel 3 to the power-generating gear 4 to develop an electromotive voltage E between respective output connections of the generator 5. When the electromotive voltage E of the generator 5 and the potential of the secondary battery have a relationship "E&gt;Vc", an electric current i1 flows through an electrical loop structured by the generator 5, a diode 12 and the secondary battery 6. An electrical energy is stored in the secondary battery 6.
On the other hand, if the handle 1 is rotated rightward as shown in FIG. 5(b), the rotation is transmitted as stated above to develop an electromotive voltage -E between the respective output connections of the generator 5. At this time, E and vc are in a relationship "-E&lt;Vc", so that no electric current i flows through the electrical loop due to the electric characteristic of the diode 12.
Incidentally, when the handle is at a standstill, the potential difference between the respective ends of the generator is at "0" and accordingly no current i flows through the electrical loop similarly to the case of the rightward rotation of the handle 1.
By providing the diode 12 in the electrical loop formed by the generator 5 and the secondary battery 6, prevention is made against reverse charging from the generator 5 to the secondary battery 6 as well as discharging from the secondary battery 6 to the generator 5.
In the small-sized generator to perform electric rectification using an electric element such as a diode, etc. as shown in the conventional example, however, if a current flows through the diode 12 in a forward direction as shown in FIG. 5(a), the electric power Wc that can be charged to the secondary battery becomes as Wc=(E-Vf)*i1. As a result, the diode 12 has an electric loss Vf*i1 occurring therein. Since the battery charger circuit has a flowing current of several hundreds mA, the forward voltage Vf becomes 0.5 V or higher when the forward current if (if=i1) exceeds 100 mA.
There is usually a necessity of "1.2 V or higher" to charge a nickel-based secondary battery, and of "3 V or higher" for a lithium-based secondary battery. Consequently, there has been a problem that, if the forward voltage vf lost by the above-state diode 12 becomes 0.5 V or higher, "the energy loss by the diode 12 with respect to the total loss amounts to approximately 30% for a nickel-based secondary battery case", and "the energy loss by the diode 12 with respect to the total loss amounts to approximately 15% for a lithium-based secondary battery case", thus lowering the charging performance corresponding to the loss by the diode.
Therefore, it is the object of this invention to obtain, in order to solve the conventionally encountered problem as stated above, an efficient battery charger which employs a mechanical mechanism to prevent, in an electrical loop formed by a generator 5 and a secondary battery 6, against reverse charging from the generator to the secondary battery 6 as well as discharging from the secondary battery 6 to the generator 5.
Also, there has been a problem that, when the generator is not in a power generating state with a handle 1 accommodated in a predetermined position, the charging button 33 is erroneously depressed to cause discharge from the secondary battery 6 to the generator.